The proposed study is designed to elucidate the interrelationships between cardiac lymph and acute myocardial infarction. To this end, we plan 1) to investigate various physiologic, pharmacologic, and pathologic mechanisms that may influence the volume and composition of cardiac lymph and 2) to determine the effects of such alterations in lymph on the subsequent changes that occur in the histologic, biochemical and electrical characteristics of the ischemic heart. In experiments already carried out (manuscript accepted for publication in the American Journal of Physiology), cardiac lymph was obtained from 12 normal dogs (Group 1) for two consecutive two-hour control periods and from 7 dogs (Group 2) for two hours before and two hours after occlusion of the circumflex branch of the left coronary artery. Lymph composition was studied with reference to pH, red blood cell (RBC) concentration, total protein content, potassium and sodium ion concentration, and creatine phosphokinase (CPK) and acid phosphatase enzyme activities. No significant difference was noted in any variable between the two groups during the first two-hour period. In Group 1, no significant change occurred in any variable as a result of the passage of time alone. In Group 2, two hours of myocardial ischemia produced increases of 53.3 plus or minus 5.1% in lymph flow, 67 plus or minus 5% in protein content, and 418 plus or minus 27% in the RBC concentration, suggesting increased blood capillary permeability. Lactate rose 120.5 plus or minus 27%, potassium concentration increased 16.9 plus or minus 2.4%, acid phosphatase increased 30 plus or minus 3%, and CPK rose 61.6 plus or minus 10.9%, suggesting ischemic injury of myocardial cells. These changes in lymph were statistically significant (P less than 0.05) in contrast to the statistically insignificant changes that occurred generally in systemic venous and coronary sinus blood. It appears that analysis of cardiac lymph in acute myocardial ischemia is superior to that of blood since it is more reliable and reflects both capillary and myocardial cell abnormalities.